I just Googled writing for healing and found all sorts of interesting websites such as One year of Writing and Healing.  I really like this web site!  There are great ideas for using writing as a practice for healing and a calendar to take you through the year.  The webmistress and author, Diane Morrow, is a former physician, now English teacher.  And she has a great list of poems for healing including authors such as Mary Oliver, Marie Howe, Rumi, and Adrienne Rich.  Seeing Marie Howe’s name reminds me that Terry Gross just interviewed her on “Fresh Air” radio show on NPR–a very lovely interview.
Wandering through this rich website I also came across another interesting place, Grief, loss, and recovery.com
Here are some interesting titles like “How can grief be good when you are grieving?” “Eight ways to let go”, and many many grief poems.
Getting back to my question about what makes writing healing, I think an important element in the mix is getting what is inside of you out and down on paper or on a computer screen.  Once you have words on some form of a page, they can be put away, read out loud, sent to others, or saved to be read later.  Whatever you decide to do with your writing, the page can become your safe container that holds whatever you need to say just like a safe with a combination lock holds valuables.